The present disclosure relates to a radio frequency power amplifier for power amplification of a radio frequency signal.
A digital mobile telephone terminal includes a multimode system which allows the terminal to be used in many parts of the world (e.g., GSM: Global System for Mobile communications, UMTS: Universal Mobile Telecommunications System). In the mobile telephone terminal, a power amplifier for high-output power amplification is provided in which, typically, two or three compound semiconductor transistors for radio frequency amplification are connected in a linear fashion. As the compound semiconductor transistor, for example, a heterojunction bipolar transistor (HBT) made of GaAs is used, in view of single positive power supply operation or the like. In recent years, in view of a reduction in size of the mobile telephone terminal, there has been progress in developing a multimode power amplifier which can be used in multiple modes.
The power amplifier in the mobile telephone terminal has more than about half the power consumption of the mobile telephone terminal, and therefore, it is essential to enable the power amplifier to operate with low power consumption so as to increase the talk time of the mobile telephone terminal.
The output power of the power amplifier is generally about +34 dBm in the GSM standard, and within the wide range of about +27 dBm to about −50 dBm in the UMTS standard. Particularly, the power consumption is highest in the vicinity of the highest output power, i.e., +34 dBm (GSM) or +27 dBm (UMTS). Therefore, it is necessary to suppress the power consumption in this vicinity.
A final-stage amplifier of amplifiers for a mobile telephone having an output power of about 300 mW to about 3 W has a configuration in which a plurality of transistors are connected in parallel and the outputs of the transistors are combined, so as to obtain radio frequency characteristics and a high output. An example configuration of such a conventional radio frequency power amplifier is shown in FIG. 13 (see Patent Documents 1 to 4).
In the conventional radio frequency power amplifier 100 of FIG. 13, a direct-current bias voltage input from a bias circuit B1 to a bias voltage input terminal DCIN is supplied to the bases of transistors Q101 to Q10n via respective resistors Ra101 to Ra10n, where n is an integer of 2 or more. Also, a radio frequency signal input to a radio frequency signal input terminal RFIN is supplied to the bases of the transistors Q101 to Q10n via respective capacitors C101 to C10n. The collectors of the transistors Q101 to Q10n are commonly connected and coupled to a radio frequency signal output terminal RFOUT. The emitters of the transistors Q101 to Q10n are each grounded (see Patent Documents 1, 2 and 4).
The bias circuit B1 of FIG. 13 includes a transistor Q0 whose collector is connected to a power supply VDC and performs emitter follower operation, and a temperature compensation circuit T1 which is connected to a power supply VREF. The temperature compensation circuit T1 includes a resistor R0, and diodes D1 and D2 (see Patent Document 3).
A reason why a bias voltage and a radio frequency signal are input to the bases of the transistors Q101 to Q10n via respective different paths as shown in FIG. 13, is as follows. Specifically, when the transistors Q101 to Q10n perform high-output operation, the current density of alternating current increases, resulting in heat generation. The generated heat is not uniform in all of the transistors Q101 to Q10n due to variations in characteristics between the transistors Q101 to Q10n. Therefore, a specific transistor whose temperature becomes high is likely to run away due to a large amount of heat generated during operation, resulting in an increase in base current, which leads to device breakdown. Therefore, in order to suppress the thermal runaway, when the base voltages of the transistors Q101 to Q10n increase, the values of the resistors Ra101 to Ra10n are increased so as to reduce the base bias current supplied from the bias circuit B1.
On the other hand, in the UMTS standard, a Probability Density Function (PDF) indicating the frequency of use in the output power of the power amplifier is highest within the range of +5 dBm to +15 dBm with its peak being located in the vicinity of a relatively low output of +10 dBm. Although the power consumption is not very high when the output power is within this range as compared to that during the highest-output operation, it is important to reduce the power consumption as well, since the frequency of use is high. Therefore, in the mobile telephone terminal, a DC-DC converter is used to control the collector voltage of the power amplifier within the range of 1.0 V to 3.35 V. Particularly, when the output power is +15 dBm or less, the collector voltage is set to 1.0 V so as to reduce the power consumption. Moreover, since a collector current in the vicinity of a low output also has a significant influence on the power consumption of the power amplifier, the current of the bias circuit B1 needs to be set to be as low as possible.
In the conventional radio frequency power amplifier 100 of FIG. 13, a capacitor CZ1 is coupled between the bias voltage input terminal DCIN and the radio frequency signal input terminal RFIN in addition to the aforementioned configuration, whereby gain compression accompanying an increase in input radio frequency signal can be suppressed, resulting in low distortion operation of the radio frequency power amplifier 100. As a result, the current of the bias circuit B1 is set to be low while a high output is achieved, i.e., satisfactory radio frequency characteristics are achieved (see Patent Document 2).    Patent Document 1: U.S. Pat. No. 5,608,353    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-324325    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2007-288736    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2003-243942